Optical micrography, electron back scattered diffraction system, microhardness tester and universal material testing machine were used to detect and analyze the cracked T91/TP347H dissimilar steel weld joint of the reheater in a 350 MW supercritical boiler. Results show that, the crack was caused by type Ⅳ cracking of new type martensite heat resistant steel and the cracking region located in the intercritical HAZ of the T91 side in the dissimilar steel weld joint. The dissimilar steel weld joint withstood relatively higher stress during the service process, while the intercritical HAZ possesses the lowest creep strength due to its microstructure feature. Crack was formed by the mode of creep void interconnection in the intercritical HAZ during the high temperature service process and led to the final cracking.